Always On VPN IKEv2 Load Balancing with Citrix NetScaler ADC

Always On VPN SSTP Load Balancing with Citrix NetScaler ADCThe Internet Key Exchange version 2 (IKEv2) VPN protocol is the protocol of choice when the highest level of security is required for Always On VPN connections. It uses IPsec and features configurable security parameters that allow administrators to adjust policies to meet their specific security requirements. IKEv2 is not without some important limitations, but organizations may insist on the use of IKEv2 to provide the greatest protection possible for remote connected clients. Due to complexities of the IKEv2 transport, special configuration on the Citrix ADC is required when load balancing this workload.

Special Note: In December 2019 a serious security vulnerability was discovered on the Citrix ADC that gives an unauthenticated attacker the ability to arbitrarily execute code on the appliance. As of this writing a fix is not available (due end of January 2020) but a temporary workaround can be found here.

Load Balancing IKEv2

When an Always On VPN client establishes a connection using IKEv2, communication begins on UDP port 500, but switches to UDP port 4500 if Network Address Translation (NAT) is detected in the communication path between the client and the server. Because UDP is connectionless, custom configuration is required to ensure that VPN clients maintain connectivity to the same backend VPN server during this transition.

Initial Configuration

Load balancing IKEv2 using the Citrix ADC is similar to other workloads. Below are specific settings and parameters required to load balance IKEv2 using the Citrix ADC.

Note: This article is not a comprehensive configuration guide for the Citrix ADC. It assumes the administrator is familiar with basic load balancing concepts and has experience configuring the Citrix ADC.

Service Settings

The load balancing services for IKEv2 VPN will use UDP ports 500 and 4500. Create the service group and assign group members for UDP 500 as follows.

Always On VPN IKEv2 Load Balancing with Citrix NetScaler ADC

Always On VPN IKEv2 Load Balancing with Citrix NetScaler ADC

Repeat the steps above to create the service group for UDP port 4500.

Virtual Server Settings

Two virtual servers are required, one for UDP port 500 and one for UDP port 4500. Ensure that the service group using UDP port 500 is bound to the virtual server using the same port.

Always On VPN IKEv2 Load Balancing with Citrix NetScaler ADC

Always On VPN IKEv2 Load Balancing with Citrix NetScaler ADC

Repeat the steps above to create the virtual service for UDP port 4500.

Service Monitoring

Since IKEv2 uses the UDP protocol, the only option for service monitoring is to use PING, which is configured by default. Ensure that the firewall on the VPN server allows inbound ICMPv4 and ICMPv6 Echo Request. The default PING monitor on the Citrix ADC will ping the resource every 5 seconds. If a different interval is required, the administrator can edit the PING monitor and bind that to the service or service group as necessary.

Persistency Group

A Persistency Group on the Citrix ADC will be configured to ensure that IKEv2 VPN client requests from the same client are always routed to the same backend server. Follow the steps below to create a Persistency Group and assign it to both IKEv2 virtual servers created previously.

1. In the Citrix ADC management console expand Traffic Management > Load Balancing > Persistency Groups.
2. Click Add.
3. Enter a descriptive name for the Persistency Group.
4. Select SOURCEIP from the Persistence drop-down list.
5. Next to the Virtual Server Name section click the Add button.
6. Add both previously configured IKEv2 virtual servers for UDP 500 and 4500.
7. Click Create.

Always On VPN IKEv2 Load Balancing with Citrix NetScaler ADC

Additional Information

Windows 10 Always On VPN SSTP Load Balancing with Citrix NetScaler ADC

Windows 10 Always On VPN IKEv2 Features and Limitations

Windows 10 AlWAYS On VPN and IKEv2 Fragmentation

Windows 10 Always On VPN IKEv2 Security Configuration

Windows 10 Always On VPN Certificate Requirements for IKEv2

Always On VPN IKEv2 Policy Mismatch Error

Always On VPN IKEv2 Policy Mismatch ErrorThe Internet Key Exchange version 2 (IKEv2) VPN protocol is the protocol of choice for Windows 10 Always On VPN deployments where the highest levels of security and assurance are required. However, as I’ve written about in the past, often the default IKEv2 security settings are less than desirable. Before using IKEv2 VPN in a production environment the administrator will need to update these security settings accordingly.

Connection Failure

When configuring Windows Server Routing and Remote Access Service (RRAS) or a third-party VPN appliance to support IKEv2 using custom security policies, the administrator may encounter a scenario in which a connection cannot be established due to a policy mismatch error. When the connection attempt fails, an error will be recorded in the Windows Application event log from the RasClient source with Event ID 20227. The error message states the following:

“The user [username] dialed a connection named [connection name] which has failed. The error code returned on failure is 13868.”

Always On VPN IKEv2 Policy Mismatch Error

Error Code 13868

Error code 13868 translates to ERROR_IPSEC_IKE_POLICY_MATCH. Essentially this error indicates that the IKEv2 security policy on the client did not match the configuration on the server.

Server Configuration

To view the current IKEv2 IPsec policy configuration, open an elevated PowerShell command window and run the following command.

Get-VpnServerIPsecConfiguration

Always On VPN IKEv2 Policy Mismatch Error

Client Configuration

To ensure interoperability, the VPN client must be configured to use the same IKEv2 security policy as defined on the sever. To view a VPN client’s currently configured IKEv2 security policy, open an elevated PowerShell command window and run the following command.

Get-VpnConnection -Name [connection name] | Select-Object -ExpandProperty IPsecCustomPolicy

Always On VPN IKEv2 Policy Mismatch Error

Note: If this PowerShell command returns no output, the VPN connection is not using a custom IKEv2 IPsec security policy.

Updating Settings

Guidance for configuring IKEv2 security policies on Windows Server RRAS and Windows 10 can be found here.

Summary

IKEv2 policy mismatch errors can be resolved easily by ensuring both the VPN server and client are configured to use the same IPsec security policies. Use the PowerShell commands in the above referenced above to validate settings and make changes when necessary.

Additional Information

Windows 10 Always On VPN IKEv2 Security Configuration

Windows 10 Always On VPN IKEv2 Features and Limitations

Show-VpnConnectionIPsecConfiguration PowerShell script on Github

Set-IKEv2SecurityBaseline PowerShell script on Github

Always On VPN IKEv2 Features and Limitations

Always On VPN IKEv2 Features and LimitationsThe Internet Key Exchange version 2 (IKEv2) VPN protocol is a popular choice for Windows 10 Always On VPN deployments. IKEv2 is a standards-based IPsec VPN protocol with customizable security parameters that allows administrators to provide the highest level of protection for remote clients. In addition, it provides important interoperability with a variety of VPN devices, including Microsoft Windows Server Routing and Remote Access Service (RRAS) and non-Microsoft platforms such as Cisco, Checkpoint, Palo Alto, and others.

IKEv2 Limitations

IKEv2 is clearly the protocol of choice in terms of security. It supports modern cryptography and is highly resistant to interception. It’s not without some operational challenges, however. Consider the following.

Firewalls

IKEv2 uses UDP ports 500 and 4500 for communication. Unfortunately, these ports are not always open. Often, they are blocked by network administrators to prevent users from bypassing security controls or attackers from exfiltrating data.

Fragmentation

IKEv2 packets can become quite large at times, especially when using client certificate authentication with the Protected Extensible Authentication Protocol (PEAP). This can result in fragmentation occurring at the network layer. Unfortunately, many firewalls and network devices are configured to block IP fragments by default. This can result in failed connection attempts from some locations but not others.

Load Balancing

Load balancing IKEv2 connections is not entirely straightforward. Without special configuration, load balancers can cause intermittent connectivity issues for Always On VPN connections. Guidance for configuring IKEv2 load balancing on the Kemp LoadMaster and the F5 BIG-IP can be found here:

IKEv2 Fragmentation

IKEv2 fragmentation can be enabled to avoid IP fragmentation and restore reliable connectivity. IKEv2 fragmentation is supported in Windows 10 and Windows Server beginning with v1803. Guidance for enabling IKEv2 fragmentation on Windows Server RRAS can be found here. Support for IKEv2 fragmentation on non-Microsoft firewall/VPN devices is vendor-specific. Consult with your device manufacturer for more information.

IKEv2 Security and RRAS

Be advised that the default security settings for IKEv2 on Windows Server RRAS are very poor. The minimum recommended security settings and guidelines for implementing them can be found here.

IKEv2 or TLS?

IKEv2 is recommend for deployments where the highest level of security and protection is required for remote connections. In these scenarios, the sacrifice of ubiquitous availability in favor of ultimate security might be desired.

SSTP or another TLS-based VPN protocol is recommended if reliable operation and connectivity are desired. SSTP and TLS VPNs can be configured to provide very good security by following the security and implementation guidelines found here.

IKEv2 with TLS Fallback

In theory, preferring IKEv2 and falling back to the Secure Socket Tunneling Protocol (SSTP) or another TLS-based VPN protocol when IKEv2 is unavailable would seem like a logical choice. This would ensure the highest level of protection, while still providing reliable connectivity. Unfortunately, the Windows VPN client doesn’t work this way in practice. Details here.

Additional Information

Windows 10 Always On VPN IKEv2 Load Balancing with F5 BIG-IP

Windows 10 Always On VPN IKEv2 Load Balancing with Kemp LoadMaster

Windows 10 Always On VPN IKEv2 Fragmentation

Windows 10 Always On VPN IKEv2 and SSTP Fallback

Windows 10 Always On VPN IKEv2 Security Configuration

Windows 10 Always On VPN Certificate Requirements for IKEv2

Windows 10 Always On VPN Protocol Recommendations for Windows Server RRAS

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